The present invention is directed to nucleic acid and amino acid sequences and constructs, and methods related thereto.
Through the development of plant genetic engineering techniques, it is possible to produce a transgenic variety of plant species to provide plants which have novel and desirable characteristics. For example, it is now possible to genetically engineer plants for tolerance to environmental stresses, such as resistance to pathogens and tolerance to herbicides. Another important example for such plant genetic engineering techniques is the production of valuable products in plant tissues, such as improved fatty acid compositions.
There is a need for improved means to obtain or manipulate fatty acid compositions, from biosynthetic or natural plant sources. For example, novel oil products, improved sources of synthetic triacylglycerols (triglycerides), alternative sources of commercial oils, such as tropical oils (i.e., palm kernel and coconut oils), and plant oils found in trace amounts from natural sources are desired for a variety of industrial and food uses.
To this end, the triacylglycerol (TAG) biosynthesis system in mammalian tissues, yeast and plants has been studied. In the cytoplasmic membranes of plant seed tissues which accumulate storage triglycerides (xe2x80x9coilxe2x80x9d), fatty acyl groups are added sequentially by specific acyltransferase enzymes to the sn-1, sn-2 and sn-3 positions of glycerol-3-phosphate (G3P) to form TAG. This pathway is commonly referred to as the Kennedy or G3P pathway (FIG. 9).
The first step in TAG formation is the acylation of the sn-1 position of glycerol-3-phosphate (G-3P), catalyzed by glycerophosphate acyltransferase (GPAT), to form lysophosphatidic acid (LA). The lysophosphatidic acid is subsequently acylated at the sn-2 position by lysophosphatidic acid acyltransferase (LPAAT) to create phosphatidic acid.
A key step in the formation of TAG is the dephosphorylation of the sn-3 position of phosphatidic acid (PA) to form sn-1,2-diacylglycerol (DAG) and inorganic phosphate catalyzed by the enzyme phosphatidic acid phosphatase (PAP, EC 3.1.3.4).
The sn-1,2-diacylglycerol is acylated at the sn-3 position by diacylglycerol acyltransferase ultimately forming triacylglycerol (TAG).
The dephosphorylation of phosphatidic acid by PAP is considered to be the rate limiting step of triacylglycerol biosynthesis in animal tissues (Brindley, (1978), Int. J. Obes. 2:7-16). Furthermore, in microsomal preparations from developing cotyledons of safflower and sunflower, the inability to form diacylglycerol from phosphatidic acid in reactions of glycerol phosphate and acyl-CoA suggests that PAP may also be the rate limiting step in plants (Stymne, et al., (1987), The Biochemistry of Plants, 9:192-193).
Phosphatidic acid phosphatase is also important in the synthesis of substrates involved in the biosynthesis of important membrane phospholipids as DAG can be converted to phosphatidylethanolamine (PE) and phosphatidylcholine (PC) via the CDP-ethanolamine (CDP-Etn)and CDP-choline-based kennedy pathway (Kennedy, et al. (1956) J. Biol. Chem. 222:193-214).
In addition, in mammalian cells, PAP is thought to be involved with cellular signal transduction to control the balance between diacylglycerol and phosphatidic acid, which are both secondary messengers in mammalian cell systems.
The characterization of phosphatidic acid phosphatase (also known as PAP) from plants is useful for the further study of plant fatty acid synthesis systems and for the development of novel and/or alternative oils sources. Studies of plant mechanisms may provide means to further enhance, control, modify, or otherwise alter the total fatty acyl composition of triglycerides and oils. Furthermore, the elucidation of the factor(s) critical to the natural production of triglycerides in plants is desired, including the purification of such factors and the characterization of element(s) and/or cofactors which enhance the efficiency of the system. Of particular interest are the nucleic acid sequences of genes encoding proteins which may be useful for applications in genetic engineering.
Stymne, et al., (1987), The Biochemistry of Plants, 9:192-193 describes phosphatidic acid phosphatase as a rate limiting step in the production of triacylglycerol in plants. Brindley, (1978), Int. J. Obes. 2:7-16 describes the rate limiting step of triacylglycerol biosynthesis in animals as the dephosphorylation of phosphatidic acid. Kai, et al. (1996), J. Biol. Chem., 271:18931-18938, describes the cloning of a gene encoding a mouse plasmalemma form of phosphatidic acid phosphatase. Berg, et al. (1997) Biochemica et Biophysica Acta, 1330:225-232 describes the purification of a phosphatase which hydrolyzes phosphatidic acid from Acholeplasma laidlawii. Carman (1997) Biochemica et Biophysica Acta 1348:45-55 describes phosphatidate phosphatases in Saccharomyces cerevisiae and Escherichia coli. 
Th present invention provides nucleic acid sequences encoding for proteins which catalyze the dephosphorylation of phosphatidic acid (PA) to form sn-1,2-diacylglycerol (DAG). Such proteins are referred to herein as phosphatidic acid phosphatase (EC 3.1.3.4) or PAP.
By this invention, nucleic acid sequences encoding plant PAP may now be characterized with respect to enzyme activity. In particular, isolation of nucleic acid sequences encoding for PAP from Arabidopsis, Brassica, soybean and corn are provided.
Thus, this invention encompasses plant PAP nucleic acid sequences and the corresponding amino acid sequences, and the use of these nucleic acid sequences in the preparation of oligonucleotides containing PAP encoding sequences for analysis and recovery of plant PAP gene sequences. The plant PAP encoding sequence may encode a complete or partial sequence depending upon the intended use. All or a portion of the genomic sequence, or cDNA sequence, is intended.
Of special interest are recombinant DNA constructs which provide for transcription or transcription and translation (expression) of the plant PAP sequences. In particular, constructs which are capable of transcription or transcription and translation in plant host cells are preferred. For some applications a reduction in plant PAP may be desired. Thus, recombinant constructs may be designed having the plant PAP sequences in a reverse orientation for expression of an anti-sense sequence or use of co-suppression, also known as xe2x80x9ctranswitchxe2x80x9d, constructs may be useful. Such constructs may contain a variety of regulatory regions including transcriptional initiation regions obtained from genes preferentially expressed in plant seed tissue. For some uses, it may be desired to use the transcriptional and translational initiation regions of the PAP gene either with the PAP encoding sequence or to direct the transcription and translation of a heterologous sequence.
In yet a different aspect, this invention relates to a method for producing a plant PAP in a host cell or progeny thereof via the expression of a construct in the cell. Cells containing a plant PAP as a result of the production of the plant PAP encoding sequence are also contemplated herein.
In addition, methods for increasing oil content in developing seed as well as methods for producing novel oil compositions in developing seeds of oil producing plants are contemplated.
Also considered in this invention are the modified plants, seeds and oils obtained by expression of the plant PAP sequences and proteins of this invention.